Videos - Flathead Bogging Down - Briggs 5hp

[JB Boozer]

Photos and Video:

https://onedrive.live.com/redir?resid=EF997A0CE0CD14D9!2505&authkey=!AAhWUVtzX99_d1U&ithint=album%2c

This is a Briggs 5hp Flathead (RaptorIII) from '99, i raced it about half the season and it's been inside dry storage ever since...it ran fine when I put it up…so about 15hrs of use, 15 yrs ago.

Now I have it back, and problem is it seems to be bogging or loosing power upon the clutch engagement, it won't hook up accelerates very slowly, and is overheating the clutch.

Without any load, when I give a small amount of throttle, it hesitates and grumbles just above idle, but it will rev freely after that.

But under load (when I’m riding), the RPM seems to go just above clutch engagement and hovers there quite a bit, just doesn't break through and hook up. The kart accelerates slowly, eventually coming up to speed. (See Video in link)
Also by looking at the video, seems like alot of ping/popping going on, maybe detonation? Or that could just be loose chain skipping sprokets…I can’t slide the engine fwd any more, so I need to get a chain breaker and remove a couple links.

Initially I thought this was just clutch slipping, but now seems like power issue...please give some advice!

Carb is cleaned, fuel, oil and spark plug are new, and everything else is OE.

Some prior suggestions were: sticky valves, or worn spark plug wire.
I've pulled the head to check the valves, both seem tight. The intake doesn't tighten up fully until compression stroke is about halfway up - i believe that is a pressure relief that is designed into the camshaft but I could be wrong there.

I went ahead and sprayed a generous amount of WD-40 down the valve stems.

With the valve lifted, they do turn but takes some effort, it's not very slippery...lubing them up seemed to help some - i'm not sure how tight or loose they should be, but I understand that under operation they're supposed to rotate around on their own.

The valve timing seems OK to me by looking at it.

Also located a rough spot on the spark plug wire under the blower cover, (photo and video of that too)... . It doesn't look like much, but i suppose it doesn't take much for Mr Sparky to escape. I'll try to test with some soapy water spray today, and/or put a couple layers of shrink tubing over it.

Also will try to get some video of startup/idle/RPM ranges.

If you guys can think of anything else please let me know! I mean basically it seems like it runs fine until i actually get on and ride it...it seems to have good spark, compression, gearing is good...i'm stumped!

Again, all the photos and videos are located here:
https://onedrive.live.com/redir?resid=EF997A0CE0CD14D9!2505&authkey=!AAhWUVtzX99_d1U&ithint=album%2c

 

[OzFab]

Raise the rear of the kart & run the engine as normal; what happens?

I'm thinking this has nothing to do with the engine & more to do with the gearing; can you please provide details of sprocket sizes (tooth count) & tyre height...

 

[KartFab]

Like Fabroman said, it may be geared too high.

Also your clutch doesn't seem to be engaging fully.

 

[JB Boozer]

14 sprocket clutch, 56 rear. ..wka sprint racing kart from back then, gearing is fine..New clutch does the same thing...and yes its smoking the clutch

 

[Poboy kartman]

14 sprocket clutch, 56 rear, gearing is smoking the clutch

There.....I fixed that for ya.

 

[OzFab]

Gearing is not "fine", 4:1 is too high...

The fact that the clutch doesn't lock combined with the fact that the clutch is smoking are both indicators the the gearing is too high...

 

[JB Boozer]

Sorry, rear is 59,
Tires are 11.5x 6.00-6 Dunlop RH7
Clutch is noram ge

Like I said, it's set up for wka sprint, put up wet too many years ago. ..gearing isn't the issue, ran fine back in the day Barnesville GA...so to be clear, it's not a yard kart, it's a sprint kart, originally setup by a racing kart shop, and I used the same clutch and gearing setup in actual wka kart racing competition...stock flathead restrictor plate, gas.

Tires off the ground, at idle tires don't move, but with throttle it spins up fine without any load, but it seems to bog on clutch engagement when riding it.

We're looking more the direction of an engine stored indoors and under tarp, but not climate controlled , rotated regularly but not cranked for 15 years...

Starting to look like could be diaphragm issue...Although the upper bowl in the tank seems to have plenty of fuel when I pull the carb, and the diaphragm seems fine, maybe it's not keeping up with the demand when under load and the fuel level is getting low in the bowl and that's causing a lean scenario...I'm not sure if that's plausible or not...does the pulsajet require a full bowl, ie less than full will cause a lean condition?

When I took the carb apart it was actually very clean, the ports weren't gummed with varnish, the diaphragm was pliable etc...The engine doesn't hunt , surge, or smoke and the upper bowl always seems to have what I figure to be plenty of gas, so I figured the diaphragm was fine...but then again maybe it fills back up at idle before I shut it off.

 

[KartFab]

JB, I really like the 5 hp flatheads. I ended up putting a different carb on mine though.

 

[JB Boozer]

KartFab - yeah they're pretty cool and simple - i'm glad I pulled this thing back out to piddle around with.
I'd like to get it in good enough shape to take racing at Atlanta Motorsports Part - about 20 mins from me. They said I can mix in with the LO206 guys so I figure to possibly put in a mild cam w/springs, smooth out the eyebrows, and possibly a tillison carb at some point - keep the stock piston/rod etc - just enough to be on par with them. If that works out I may eventually get a LO206 and find a mini-bike to put the flathead on, or a larger yard kart to cruise my kids around the neighborhood.

I appreciate the help I get on the forums - cool youtube channel you've got, subscribed.
Hey have you ever seen a kart kit that has a bit wider seating...i'm thinking driver in the middle with a kid on either side - I have twins 6yo.

 

[JB Boozer]

update - I replaced the diaphragm over the holidays, and still did not help.
also, just to revisit on the gearing
Clutch 14t; sprocket 59t, tires 11.5"

I did try a new clutch with 12t, and same thing happened.

Also the folks at Atlanta Motorsports Park have recommended a 17t-19t clutch with the 59t rear sprocket...and that track has about 42' of elevation change, with turn 2 being basically 180 degree hairpin located inside a 40' hole in the ground ;-) but the track is almost a mile long with loooong straights that require this longer gearing...the track manager is recommending that gear based on current LO206 racers.

My old track was Lamar County Speedway in Barnesville GA, a short 0.25mi track with very little elevation change and a medium length straights, so my 14t/59t is actually supposed to be a kind of a short gearing setup to just hit max rpm at the end of those straights, but accelerate strongly out of the corners.

I invite you guys to check out the link with all the pics and videos. The engine itself seems to be bogging around the clutch engagement point and there are some audible pop/pings going on that don't seem to be backfire...

While I do agree that gearing would be one of the first culprits to investigate, given the above explanation I believe the solution is more likely to be found elsewhere.

Thanks in advance for any assistance!

https://onedrive.live.com/redir?resid=EF997A0CE0CD14D9!2505&authkey=!AAhWUVtzX99_d1U&ithint=album%2c

 

[KartFab]

did you check compression yet with an actual compression tester? What about setting the valve lash with feeler gauge?

 

[Kokamo]

I agree with the 3 guys above. Gearing is your problem. The reason it boggs from a stop until it gets some speed and keeps the same RPM (noticed it in the video) is because the clutch is slipping until the ground speed is matched to full clutch engagement speed. This is why your clutch is smoking...that means the motor is making power, it's just that the inability to turn the clutch is whats keeping the motor from reaching it's proper powerband. 4.21 gearing (14tx59t) with an 11" tire and 5hp should pull about 65 to 70lbs (kart and driver) properly to almost 30mph. The 12t clutch should do better, but still yields a 4.9 gear ratio and 25mph. Ideally, get a 10t clutch....would be 6.0 gear ratio and that would be most likely the best way to go. Go HERE and read up...I'm betting your current gearing is the culprit. If it ran fine and worked fine 15 years ago, maybe you weighed less then?? (no offence at all )

~Joe

 

[landuse]

Also the folks at Atlanta Motorsports Park have recommended a 17t-19t clutch with the 59t rear sprocket

This will make it even worse. Do not even consider it. The people at the racetrack should know better

 

[KartFab]

Ok guys just a quick bit of information. Gearing that we commonly see for yard karts is not applicable for race karts. Race karts are geared for high speed and few stops. Once the launch is complete, or even after the karts are going, their gearing is meant to keep them going. What we would normally see as bad gearing for a yard kart is excellent for racing because the clutch isn't always engaging/disengaging from stop or slow.

 

[Kokamo]

Ok guys just a quick bit of information. Gearing that we commonly see for yard karts is not applicable for race karts. Race karts are geared for high speed and few stops. Once the launch is complete, or even after the karts are going, their gearing is meant to keep them going. What we would normally see as bad gearing for a yard kart is excellent for racing because the clutch isn't always engaging/disengaging from stop or slow.

I understand your logic, but I'm not under the impression that the OP is using this as a racing kart, but rather a mess around kart like most on this forum. That would mean for more decent gearing more suited for casual riding around the pasture/yard/block, etc.

~Joe

 

[JB Boozer]

yeah, i still need to check the compression with a gauge.

I did find one trick, read that if you spin the flywheel backwards and it springs back forwards, then compression is likely not the issue...so that's all I've tried and it did bounce back very well...idk how effective that test is though.

For Valve lash...idk, is that going to go south just sitting around? What would be wrong there given the symptoms?

---------- Post added at 06:42 PM ---------- Previous post was at 06:12 PM ----------

Yes Joe, it is intended eventually for racing, but first for blasting around the neighborhood to shake out the bugs...and yet it's not even really getting going from a stop

I did used to weigh about 160 and now about 190 so yeah i've put on some, however these classes are setup to be min. weight specific and some folks have 30#s of lead attached to their karts and still gearing them to the length of the straight vs max RPM, so I'm just guessing it shouldn't matter(?)
heck i've seen videos of huge 300# guys hopping onto thin 120# guys mini-bikes and not having a clutch/gearing slip issue.

This is the track I used to race at, and even though these aren't flatheads, it's exactly what it used to be like ie punch the gas and it just takes off...but the way my kart is running now, it wouldn't even make it out of the S turns, much less start off the line.
https://youtu.be/r0NMW2Mts7Q?t=18s

The track I'm aiming for is AMP:
https://youtu.be/TTL2LHBL-YI?t=30s

As you can see, there is more bogging on the start (presumably due to taller gearing), much longer straights (longer gear is needed), and much slower corner with massively steep hill to climb out of (turn 2) so currently my kart is in no shape at all for this. these are factory sealed LO206 class engines, pretty close in power to what I have now. This is the track they're recommending a 17 or 19 clutch x 59 sprocket gearing...and obviously if I'm not geared right, I'll be several mph slower in the straights.

I did try a new 12t clutch and it made absolutely no difference.

 

[JB Boozer]

So I have to say, there seems to be alot of popping going on during the video I posted....any ideas on what that could be?

 

[JB Boozer]

Update, did the compression test, came back with 60psi...i did try putting some oil into the cylinder and trying again, same ~60psi...someone saying possibly a stuck piston ring.

also, another person has indicated he feels my timing may be off, based on my photo of flywheel at TDC...says appears slightly retarded timing

So, top contenders are timing and stuck ring...will have to pull the flywheel off first to check timing.

 

[landuse]

60 is a bit low

 

[JB Boozer]

someone else was saying 60psi may be ok with the ez-start compression release built into the cam...but idk, seems low to me too

Checked flywheel timing key today, looks straight on.
Guess it's time to crack open - hopefully more interesting things going on in there